Monday, March 14, 2011

85 Octane?

Was surprised to see 85 octane gasoline for sale at a gas station in Utah.  The Wikipedia entry on octane ratings explained the mystery.  Due to the lower atmospheric pressure at higher elevations, an engine designed to use 87 at sea level can get by on 85, or in Wikipedia's words:

United States: in the Rocky Mountain (high elevation) states, 85 AKI is the minimum octane, and 91 AKI is the maximum octane available in fuel[citation needed]. The reason for this is that in higher-elevation areas, a typical naturally-aspirated engine draws in less air mass per cycle because of the reduced density of the atmosphere. This directly translates to less fuel and reduced absolute compression in the cylinder, therefore deterring knock. It is safe to fill a carbureted car that normally takes 87 AKI fuel at sea level with 85 AKI fuel in the mountains, but at sea level the fuel may cause damage to the engine. A disadvantage to this strategy is that most turbocharged vehicles are unable to produce full power, even when using the "premium" 91 AKI fuel. In some east coast states, up to 94 AKI is available [2]. In Colorado as well as parts of the Midwest (primarily Minnesota, Iowa, Illinois and Missouri) ethanol-based E-85 fuel with 105 AKI is available [3]. Often, filling stations near US racing tracks will offer higher octane levels such as 100 AKI[citation needed] . California fuel stations will offer 87, 89, and 91 AKI octane fuels, and at some stations, 100 AKI or higher octane, sold as racing fuel. Until summer 2001 before the phase-out of methyl tert-butyl ether aka MTBE as an octane enhancer additive, 92 AKI was offered in lieu of 91.

From Innovation Bootcamp

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